The invention relates to a noise detector for detecting noise pulses in a non-limited incoming radio signal modulated on a carrier, comprising an amplitude detection circuit and, coupled thereto, a first signal comparison circuit having a signal input and a threshold input, a first threshold signal which depends on the average input level of the noise detector and a second threshold signal which depends on the noise pulse repetition frequency being applied to this threshold input.
Such a noise detector is disclosed in the United Kingdom Patent Specification No. 2,113,047.
In the Patent Specification the noise detector is described for use in a radio receiver whose RF-input signal is applied to the signal input of the first signal comparison circuit through an RF-amplifier. The level of the input signal at which noise detection is effected--called the threshold value hereinafter--is determined with the prior art noise detector by the sum of the first and second threshold signals. The first threshold signal is obtained by amplitude detection and a certain amount of integration of the IF-signal. As the average IF-signal varies with the average RF-input level, for example the field strength of the incoming signal, a field strength--dependent noise detection is thus obtained.
The second threshold signal is obtained by measuring the repetition frequency of the detection signals at the output of the first signal comparison circuit, the noise detection also depending on the repetition frequency of the amplitude disturbances in the input signal of the noise detector via this second threshold signal.
The derivation of the second threshold signal from the output signal of the first signal comparison circuit introduces however a control error which is necessary to preserve the control. An increase of the noise pulse repetition frequency at the input of this first circuit causes the second threshold signal to increase only when the repetition frequency of the detection signals at the output has increased. A high value of the second threshold signal, which is desired in the event of a high noise pulse repetition frequency, can however only be obtained if the repetition frequency of the detection signals is also high. In the case in which these detection signals are used as control signals for cancelling noise by means of signal suppression this may result in a fast and consequently annoying sequence of interruptions in the sound reproduction. This effect can be reduced by selecting the dependency of the total threshold level on the first threshold signal to be large compared with the second threshold signal. However, this also reduces the initial input sensitivity of the noise detector, that is to say the input sensitivity in a somewhat longer absence of noise pulses.